Usually, an aircraft taxies on the ground using drive from its thrusters. Thus, while the aircraft is taxiing on the ground, the wheels of the landing gear of the aircraft turn by freewheeling. Recently, aircraft manufacturers have been giving consideration to applying independent drive devices so as to drive some of the wheels and thus enable an aircraft to taxi on the ground without assistance from its thrusters. The landing gear wheels fitted with such devices therefore do not freewheel, but instead turn at a speed of rotation that is set by the motor of the independent drive device associated therewith, and regardless of the type of movement that the aircraft is performing on the ground.
During a turning movement on the ground, a dynamic constraint comes into play: the wheels need to turn at different speeds of rotation depending on how far away they are from the instantaneous turning center of the aircraft. When the wheels are freewheeling, this dynamic constraint does not raise a problem. In contrast, when the wheels are driven in rotation by means of independent drive devices, this dynamic constraint needs to be taken into consideration so that the independent drive devices drive the wheels to rotate at appropriate speeds.
Proposals have thus been made to calculate the appropriate speed of rotation for each wheel fitted with an independent drive device on the basis of the following relationship, giving the difference between the speeds of rotation of two wheels regardless of whether or not they are fitted with independent drive devices, while an aircraft is performing a turning movement:
      Δ    ⁢                  ω        .                    i        -        j              =                    l                  i          -          j                            R        wheel              ⁢                  φ        .            airplane      where:
Δ{dot over (ω)}i-j is the difference between the speeds of rotation of a wheel number i and a wheel number j;
li-j is the distance between wheel number i and wheel number j;
{dot over (φ)}airplane is the turning rate of the aircraft; and
Rwheel is the rolling radius of the two wheels numbers i and j (assuming that each of the wheels i and j has the same rolling radius).
It should be recalled that for a wheel, the rolling radius is the notional radius obtained by the ratio of a speed in translation of the wheel in a horizontal direction divided by the speed of rotation of the wheel.
Nevertheless, the rolling radius turns out to be complex to determine since it depends on numerous factors such as the structure of the tire of the wheel, tire wear, tire inflation pressure, . . . . A wrongly estimated rolling radius for a wheel leads to a wrong estimate for the appropriate speed of rotation for that wheel. The corresponding independent drive device thus imparts an unsuitable speed to the wheel, thereby giving rise to high levels of mechanical stress in the undercarriage that includes the independent drive device: this leads to accelerated wear of the tire and of the undercarriage and possibly also to damaging the undercarriage.
Furthermore, the independent drive device is more heavily loaded, thereby increasing the total instantaneous power developed by all of the independent drive devices. This gives rise to an increase in the power consumption of the independent drive device and to a reduction in the efficiency of the independent drive device.